A collet-type chuck is known, as for example from my German Utility Model No. 7,706,193, which has a chuck body adapted to be rotated about an axis and formed centered on this axis with a forwardly open tool-receiving seat normally of frustoconically tapered shape. A holding element is displaceable axially in this body between a rear holding position securing an appropriately tapered tool shank in the seat and a front releasting position permitting the tool to be inserted backwardly into and withdrawn forwardly from the seat. At least one holding spring is braced between the chuck body and the holding element for urging the holding element axially backwardly into the holding position.
This holding element in turn can be displaced axially forwardly to release the tool by hydraulic pressurization of a chamber formed between a rear surface of the holding member and the chuck body. Thus when the forwardly effective hydraulic pressure exceeds the backwardly effective spring force, the holding element moves forwardly to release the tool gripped by the holding element in the seat.
The problem with such a chuck is that if any axially forwardly effective force on the tool exceeds the spring force of the holding spring, the tool can be pulled out of its seat. Thus if the tool gets caught in a workpiece it can be jerked out of the chuck. Increasing the spring force makes this type of accident less likely to occur, but requires the chuck to be built more strongly and requires more hydraulic force to be employed to release the tool.
It has been suggested to provided a wedge arrangement that is axially effective between the holding spring and the piston on the holding element. Such an arrangement is disadvantageous when the holding element is used not only to secure the tool in the seat, but also to pull the tool back into the chuck since the wedge arrangement decreases the effective stroke of the piston on the holding element. In addition the extremely high axial forces exerted by such an arrangement frequently lead to deformation of the chuck body and overloading of the bearings carrying the chuck.